We have recently reported a discrepancy between the apparent rate of DNA synthesis and of thymidine accumulation by normal and stimulated murine non-adherent spleen cells. Leukemic lymphocytes also show a similar discrepancy but the magnitude may be different. The discrepancy can partly be explained by the fact that lymphocytes incorporate 3H-thymidine into a macromolecular material which is not DNA. We propose to chemically characterize the material and also to explore its physiological significance in these and other cells.